A power-generating plant often employs cooling towers in order to reduce the temperature of the coolant used in the plant. This coolant is usually water and the cooling tower is arranged to pass atmospheric air through the water so as to cool it by conduction and evaporation.
In recent times the mechanical-draft type of tower has been increasingly replaced by the so-called hyperbolic natural-draft tower. This latter structure comprises a huge vertical tube or chimney often several hundred feet high and more than a hundred feet in diameter. The tube has the shape of a generally hyperboloidal body of revolution and has a lower outwardly flared mouth spaced above the ground. Between the lower edge of this tower and the ground there is provided so-called fill through which the hot coolant is trickled. Air entering the tower through this fill and passing over the water is heated conductively, simultaneously cooling the water by evaporating some of it. The heated air inside the tower then rises by convection and draws more air in through the fill. In this manner convective flow draws a large quantity of air over the water to be cooled without the use of any external energy.
Typically such a hyperbolic (also known as parabolic) cooling tower is made of reinforced concrete. Such construction is extremely expensive, and has the further disadvantage that, because of its great weight, a very strong supporting structure must be provided. It has also been suggested to use flat metal plates welded to a frame, however this arrangement has the further disadvantage that, when steel is used, it corrodes so that its service life is relatively limited, and at the same time the considerable thermal expansion of the metal can lead to injurious deformation of the tower structure. This thermal-expansion problem is particularly aggravated because the air pressing upwardly through the interior of the tower is frequently very hot, in the neighborhood of 60.degree. -80.degree. C, whereas the outside air is frequently much colder, especially at the top of the tower which may be as many as 400 feet above the ground. Furthermore when such a tower is taken temporarily out of use it cools quickly, thereby shrinking so that any latent defects in the structure are continuously aggravated by the considerable thermal shrinkage and expansion that it is subjected to.